This invention relates to metalization for integrated circuits (ICs) and more particularly to Al-based metalization for Si ICs.
In ICs metal layers are used to make electrical contact to device active areas (e.g., as runners between devices on the same level of an IC or as vias between devices on different levels). In both cases, vias or windows with essentially vertical sidewalls are opened in an overlying dielectric layer to expose a portion of an underlying layer (e.g., the semiconductor active areas of devices or the commonly referred to "Metal-1" of the first level of metalization). A plug of metal from an overlying metal layer (e.g., "Metal-1" in the case of a device contacts or the commonly referred to "Metal-2" in the case of interconnects) extends through the via or window and makes an electrical connection between a metal layer and a semiconductor (device contact) or between two metal layers (interconnect). For simplicity, we will hereinafter refer to the electrical connection in both cases (metal to semiconductor, and metal to metal) as an interconnect.
In Si ICs aluminum alloys are the most common materials used for such metal layers. Typically, these Al-based layers are deposited as a single material by means of a single-step deposition such as sputtering.
In windows or contacts to active regions of a device, a typical metalization structure includes sequentially deposited layers of titanium (Ti), titanium nitride (TiN), and optionally another Ti layer, followed by either aluminum-silicon (Al--Si) or aluminum-silicon-copper (Al--Si--Cu) alloys (but not both). The structure is designed to prevent Si migration and junction spiking. That is, Al--Si-based alloys, with the amount of Si satisfying the solubility limit in the Al alloy, are commonly used to prevent the migration of Si from the Si body containing the devices into the Al-alloy, whereas the Ti and TiN layers serve as barrier layers in the metal interconnect to prevent Al spikes from penetrating into the Si body.
Although the use of such Si-containing Al-alloys satisfies the solubility requirements, deposition of Al--Si or Al--Si--Cu on Ti/TiN or Ti/TiN/Ti in conventional apparatus (e.g., a cluster tool) at typical temperatures (e.g., around 200.degree.-400.degree. C.), results in Si precipitation within the Al layer The Si precipitation reaction is most pronounced at the At layer free surface and occurs either during the deposition sequence or immediately upon cooling the wafer from the deposition temperature. The reaction is most pronounced in metalization containing Ti-based alloy or compound layers. This Si precipitation reaction results in significant surface roughness of the Al-alloy layer free surface, thus creating processing problems with subsequent lithography steps (patterning); that is, steppers are not able to focus well, resulting in poor printing and linewidth control. Moreover, Si precipitates at the free surface may act as nucleation sites for stress-induced voiding when the Al-based interconnect is encapsulated with a passivation layer such as a dielectric. Therefore, Si precipitation is a problem because it degrades the lithography process via surface roughness and it reduces interconnect reliability via stress-induced voiding.